Pre-heating of the combustion air by means of the heat of the flue gas is one of the standard methods to improve the capacity of a thermal power boiler. The pre-heaters for preheating combustion air can be divided into recuperative and regenerative pre-heaters. In the recuperative pre-heaters, heat is conducted through heat surfaces directly from the flue gas to the combustion air, whereas in the regenerative pre-heaters, the flue gas first heats storage mass, usually a metal or a ceramic battery, which, in the second stage, emits heat to the combustion air.
The regenerative pre-heaters are common especially in the large thermal power boilers due to their relatively small size. In the so-called rotary plane pre-heaters that are most commonly used, the heat storage mass rotates slowly, usually, 3-5 rpm in such a way that it is alternately in contact with the flue gas flow and the combustion air. In the pre-heaters for combustion air with a stationary plane, the storage mass remains still, but the connecting conduits for flue gas and combustion air comprise a rotary member, by means of which, the gases are guided to flow alternately through different sectors of the storage mass.
For example, U.S. Pat. No. 5,915,340 discloses a regenerative pre-heater for combustion air with a four-sector rotary plane, in which the flue gas flows through one flow sector, and primary air and first and second secondary air through the three other flow sectors. The rotary axis of the regenerative pre-heater for combustion air can be either horizontal or vertical, but, especially, pre-heaters with a vertical rotary axis are discussed below. The present invention is especially related to pre-heaters for combustion air, in which the flue gas flows through a pre-heater downwards and the combustion air upwards, respectively.
According to the prior art, the furnace, back pass and pre-heater for combustion air of a thermal power boiler are located one after another in the direction of the normal of the rear wall or side wall of the furnace, as disclosed, for example, in FIG. 2 of U.S. Pat. No. 5,915,340. The back pass refers, in this connection, to a vertical portion of the first flue gas channel, which usually comprises heat exchangers, such as superheaters, reheaters and pre-heaters for feed water.
In the lower part of the back pass, there is usually a turn channel, from the farthest edge of which, with respect to the furnace, the flue gas channel extends towards the pre-heater for combustion air. There is actually, at the bottom of the turn channel, an ash hopper collecting flue gas particles. Usually, the flue gas channel comprises, after the turn channel, first, a horizontal or inclined channel portion, which is directed away from the furnace, and then, a vertical channel portion, which leads to the upper portion of the pre-heater for combustion air.
When the furnace of a large thermal power boiler, back pass and pre-heater for combustion air are arranged conventionally one after another, the channels connecting them to each other can become long, whereby the design and assembly thereof among the other equipment and parts related to the furnace is complicated. Each portion and bend of the flue gas channel requires space, which could otherwise be used for locating other parts, for example, fuel treatment equipment.
In order to have different flow sectors of the pre-heater for combustion air wide enough to keep the pressure losses reasonable, the diameter of a regenerative pre-heater for combustion air to be arranged in a large thermal power boiler can be very large. It is also known that the flue gas channel of a large thermal power boiler is divided into two portions, which lead into two adjacent pre-heaters for combustion air, whereby the size of an individual pre-heater remains reasonable and the assembly thereof is relatively easy.
As the flue gas is usually guided from a pre-heater for combustion air further to the cleaning apparatuses for flue gas, for example, to a dust separator, it is natural to arrange the pre-heater in such a way that the inlet and outlet connections for the flue gas are in the portion of the pre-heater that is the farthest from the furnace. Thereby, the inlet and outlet connections for the combustion air are on the furnace side of the pre-heater, respectively. In the regenerative pre-heaters for combustion air with a rotary plane, the inlet and outlet connections for flue gas are, naturally, in the same portion of the pre-heater as the flow sector for flue gas, and the inlet and outlet connections for combustion air are in the same portion of the pre-heater as the flow sector for combustion air, respectively. In the regenerative pre-heaters with a stationary plane, the inlet and outlet connections for combustion air and flue gas are stationary, but the flow sectors for combustion air and flue gas rotate with the rotary members of the respective connecting conduits.
FIG. 1 of U.S. Pat. No. 7,438,876 discloses a conventional embodiment in accordance with the prior art, in which a regenerative pre-heater for combustion air with a rotary plane is arranged outwards from the furnace with respect to the back pass and the flow sector of combustion air in the pre-heater is in the furnace side of the pre-heater. A problem with such an arrangement is that the channel for combustion air must be bent in the next stage underneath the back pass, which, in practice, usually leads into a complicated channel structure. FIG. 9 of U.S. Pat. No. 7,438,876 discloses another possible alternative in which the flow sector of the pre-heater for combustion air is arranged away from the furnace. This arrangement is advantageous when there is a second, recuperative pre-heater for combustion air arranged above the regenerative pre-heater for combustion air, and connected in series therewith, through which pre-heater, the combustion air is guided to the side of the furnace.